The Microscope. 345 



parts changing to yellow-brown; and at last fading out to a colorless 

 base." 



The Geem Theory of the Dental Caries. — W. D. Miller, Inde- 

 pendent Practitioner, has discovered that the bacteria found in the 

 mouth and in dental cavities is an acid-producing anrerobic fungus, 

 thus accounting for the peculiar manner in which teeth are destroyed. 

 "Whenever, either between the teeth, in sulci or pockets, through faulty 

 development, a lodgment can be had for food products, a miniature 

 acid manufactory is set up. The acid freed in this manner decomposes 

 at once that portion of the tooth, with which it comes in contact, 

 uniting with its lime salts, and forming new combinations and, 

 as decalcification advances, bacteria follow after, throwing out in 

 their growth new waste products in the form of lactic acid. The 

 sugar, which is ever present in the mouth in some form or other, is 

 the natural ferment food on which these bacteria thrive, and if by 

 any chance it is not present, the ptyaline of the saliva soon produces 

 it by its action on the stai'chy elements in our daily food. Experi- 

 ments with pure cultures of this bacteria upon sound teeth showed 

 that the tooth became softened and pliable at the end of the first week, 

 and that by the close of the second week, it was completely decalcified 

 and could be easily cut with a knife. Stained sections of the tooth 

 showed all the well-known signs of dental caries to be present. 



Three separate ferments are produced by the fungi of dental 

 caries. The first, an inverting ferment, one capable of changing a 

 non- fermentable sugar, like cane sugar, into a fermentable one, and 

 possibly of forming a sugar from starch. The second, an acid pro- 

 ducing ferment, the product of which has been proved to be lactic 

 acid. The third a peptonizing or digestive ferment. All play import- 

 ant parts in the work of tooth destruction. The first prepares 

 the food for the second. The second dissolves out the lime salts of 

 the tooth, and leaves it an easy prey for the third, which, in turn, 

 eats up, dissolves, the animal basis left after the mineral matter has 

 been removed, and reduces the whole to a pulpy or fluid condition. 



Dermal Sensory Organs of Insects. — Dr. O. vom Rath, 

 {Zeitsch. f. wiss. Zool. xlvi, 1888. Jr. R. M. Soc.) 



In regard to the physiology of these organs, little is definitely 

 known, and as the structure of the various organs is essentially the 

 same, nothing can be concluded therefrom. The most important 

 position is that of the antennae. Here we find sensory hairs, cones, 



